The present invention concerns photographic cameras, most especially but not exclusively motion-picture cameras, of the type provided with an automatic focussing system which adjusts the distance-setting of the camera's exposure objective in accordance with one or another of the various techniques which have now become conventional in the art. Most typically, such systems employ one or another form of adjusting motor coupled to the exposure objective and operative when actuated for changing the distance-setting of the objective. In a simple, fully automatic focussing system, the actuating signal applied to the adjusting motor is furnished from the output of a servo comparator, one input of which receives an actual-setting signal per se indicative of the present distance-setting of the exposure objective, with the other input of the servo comparator receiving a required-setting signal supplied to it by a distance-measuring system of one or another conventional type, e.g., operating on the basis of the travel-time measurement of waves transmitted to the subject and reflected back to the camera, operating on the basis of optical triangulation methods, or whatever. With such simple, fully automatic focussing systems, whenever the system is operative the exposure objective is adjusted from whatever its present distance-setting is to a new distance-setting in response to changes in the automatically measured subject-distance. Sometimes, such systems are of only a semiautomatic character. For example, a simple, fully automatic focussing system may have a manual override feature permitting the user to manually control the exposure-objective distance-setting, e.g., with visual reference on the user's part to an automatically operating range finder or by means of subjective skill and choice alone, in which case the adjusting motor of the system receives an actuating which is per se manually varied by the user somewhat in the sense of an ordinary focus-adjuster ring, but with the actual physical change of exposure-objective distance-setting then being implemented by the adjusting motor of the manually overriden fully automatic focussing system. In addition to such simple, fully automatic focussing systems and such manually controlled but motor-adjusted focussing systems, there are also various hybrids of fully automatic and manual control. For example, in some systems, coarse or approximate focussing is performed under manual control, e.g., through the intermediary of the adjusting motor, and fine-adjustment focussing then follows in a fully automatic manner.
No matter how the actuating signal for the adjusting motor is developed, i.e., fully automatically, by means of manual selection alone, or by a hybrid of the two, the time required for the adjusting motor to bring the exposure objective to the distance-setting commanded necessarily depends upon the amount of the difference between the objective's present distance-setting and the distance-setting into which it is to be brought. If, when the user is ready to initiate a still exposure or a motion-picture exposure sequence, the present distance-setting of the exposure objective is far removed from the distance-setting into which the objective must be brought, considerable time may be required for the objective to be actually brought into the proper setting, e.g., even as long as several seconds, resulting in often important delays in the possibility of initiating well-focussed exposures and/or resulting, in motion-picture contexts, in the production of a series of quite poorly focussed exposures at the start of a motion-picture scene.
Especially when such systems are employed in motion-picture cameras, it is known to terminate operation of the adjusting motor at the termination of the exposure sequence, with the result that when the next exposure sequence is to be initiated the exposure objectivve will be at the distance-setting last used or reached at the conclusion of the previously filmed scene. This can be extremely undesirable for several reasons. Firstly, in worst-case situations, the previous distance-setting of the objective may be very far or maximally removed from the distance-setting next needed, so that the duration of the time interval required to bring the objective into the distance-setting next needed may be really very considerable. In addition to such time interval being sometimes rather lengthy, it is furthermore variable and indefinite. As a result, to the extent that the user consciously or unconsciously accustoms himself to the fact of such a time delay, he will not in general be in a position to know what the true length of the delay will be in any given situation, e.g., because despite his approximate awareness of the subject-distance presently involved he will find it impractical to attempt to remember what the previous subject-distance was. In this way, from habit or conservativeness, the user tends to accustom himself to the longest value of the time delay, because despite his awareness of the present subject-distance he has from that no real advance idea of what the amount of the time delay will be.